The idea that asymmetries may exist in the human auditory system derives largely from demonstrated functional asymmetries in auditory verbal processing at the level of the cerebral cortex. One of the challenges to modern neurology is to describe the neurobiological factors that underlie these functional asymmetries. In studying the brainstem auditory evoked potentials to clicks, we have found asymmetries for some components, which cannot be attributable to endorgan asymmetries. The nature of the brainstem asymmetries resembles that of the functional asymmetries in that right-handers show greater responses to stimuli presented to the right ear and left-handers show a more mixed patter. We propose to further study this relationship by examining how the brainstem potentials relate to handedness, dichotic listening, and verbalmanual time-sharing asymmetries. If the asymmetries in evoked potentials are related to the functional asymmetries, many possible mechanisms can be proposed. One is that asymmetrical input from lower auditory centers creates asymmetries in the brain at higher levels through selective neuronal elaboration or growth. As support for this concept we will observe whether brainstem asymmetries are present in preverbal infants. Another possible mechanism is that outputs from the higher centers (presumably asymmetric from the start) affect the brainstem asymmetrically. This second hypothesis will be tested by finding how the brainstem potentials change with alterations in the state of one of the hemispheres as occurs following unilateral stroke, with unilateral electroconvulsive therapy or during the performance of concurrent lateralized cognitive auditory tasks. Taken together these studies will provide a basis for a comprehensive account of how auditory brainstem asymmetry relates to cerebral laterality.